The present invention relates to an improvement to a reflecting mirror (hereinafter referred to as “reflector”) used for a light source of a projector such as a liquid crystal projector and an overhead projector or the like.
A projector such as a liquid crystal projector or an overhead projector or the like irradiates light from a light source onto an object (which corresponds to an image display device such as a liquid crystal panel in the case of a liquid crystal projector) and projects the light modulated by this object onto a screen, etc. using an optical device to display the image. This light source has a configuration combining a light emitting lamp and a reflector to irradiate the light of this lamp and condense it in a specific direction. As the lamp for this light source, a short-arc type metal halide lamp with a metal halide sealed in a light emitting tube and with a short inter-electrode distance was previously used. On the other hand, as the reflector of the light source, a reflector with a heat-resistant glass inner wall coated with a multi-layer film of titanium oxide and silicon dioxide was previously used. Then, the metal halide lamp was replaced by an ultra-high pressure mercury lamp, which realizes high brightness easily, or a xenon lamp, which provides high color rendering, and these are used widely. Among them, this ultra-high pressure mercury lamp improves the light emitting efficiency and realizes high brightness by elevating the vapor pressure of mercury to 200 atm or higher while the lamp is lit. Furthermore, by mixing additives other than mercury, the ultra-high pressure mercury lamp improves its spectral distribution characteristic and realizes high color rendering.
However, this high pressure mercury lamp is under severe restrictions on its operating temperature. There is also a problem that using the high pressure mercury lamp outside its optimal designed range reduces the light emitting efficiency as well as the life of the lamp tube.
The reflector used for this projector light source used to be obtained by applying press molding to heat resistant glass with a small coefficient of thermal expansion, then coating the inner wall of the reflector with an aluminum-evaporated film having a reflectance of approximately 90% and applying antioxidant treatment to this surface. In response to a demand from the market in recent years for a reflector that will realize higher brightness, a reflecting surface used for the reflector is provided with an optical multi-layer film made up of TiO2 and SiO2 capable of providing higher reflectance. Luminous flux emitted from this reflector is generally transformed to parallel or convergent luminous flux. Thus, the mainstream of the shape of the reflecting surface of the reflector is paraboloidal or ellipsoidal.
The spatial distribution of light emitted from a light source is equalized through a lighting optical system. The uniformly distributed light is irradiated onto an image display device with pixels arranged in a matrix form such as a liquid crystal panel or DMD (Digital Micro Mirror Device). The image display device forms an image based on a television signal supplied or a video signal from a computer and modulates the above-described uniformly distributed light on a pixel-by-pixel basis. The modulated light is magnified by a projection lens and projected onto a screen, etc. A display device in such a configuration with no screen is called a “projection type image projector (front projector)” and a display device with a screen is called a “rear projection type image display device”. These projection type display devices are widely spread in the market as display devices suited to providing large screens.
FIG. 1 is a sectional view of a general light source for a projector using an ultra-high pressure mercury lamp as the light source. In the case of a light emitting tube in the power consumption 100 W class, the inner volume of a quartz glass light emitting tube 1 is 55 μl, electrodes 2 are sealed at both ends and the arc length between the electrodes is set to 1 to 1.4 mm. The light emitting tube 1 contains mercury as a light emitting substance and hydrogen bromide together with argon as starting aid gases with a predetermined ratio between the two gases. A molybdenum foil 4 is welded to an electrode central axis 3, forming an electrode sealed section 5. A base 6 is attached to the electrode sealed section 5 on the reflector bottom opening side. This base 6 is adhered or fixed to, through cement 8, the bottom of a reflector 7, on the inner surface of which a multi-layer reflection film is formed so that visible light is reflected and infrared rays are allowed to pass. In this case, the base 6 is fixed in such a way that the quasi-focal point of the reflector lies on the extension of the arc axis of the light emitting tube 1. Front plate glass 9 having almost the same coefficient of thermal expansion as that of the reflector 7 is set in the flange section of the front opening of this reflector 7. In the event of a burst of the light emitting tube, this front plate glass 9 is intended to prevent fragments of the light emitting tube from flying in all directions and reflection preventive coating is applied to both sides of the front plate glass 9.
FIG. 2 shows a mode of use of the projector light source shown in FIG. 1 when it is used as the light source for an actual optical apparatus such as a liquid crystal projector or overhead projector. In FIG. 2, the same components as those in FIG. 1 are assigned the same reference numerals and explanations thereof are omitted.
A cooling fan 10 is set on one side of or behind the projector light source and a desired cooling effect can be obtained by blowing air toward the reflector 7. Another method is to suction the air around the light source heated by lighting of the lamp and thereby produce an air flow to cool the reflector.